Skin and gut: what there is to the gut-skin axis

Healthy state

Skin and gut have a lot in common: both are borders with the outside world, both are densely colonised by microbes, and at both the immune system learns to tolerate harmless things. Researchers speak of a gut-skin axis. The idea is that substances from food and gut bacteria, and immune cells from the gut, can also influence the skin. So far this is mainly a model; which pathways really matter in humans is only partly understood.

The link is clearest in diseases that affect both organs. Inflammatory bowel disease Inflammatory bowel disease Umbrella term for Crohn’s disease and ulcerative colitis: long-term, relapsing inflammation of the gut that needs medical treatment. More in the chapter “Immune system and inflammation” can show up on the skin, for example as painful red lumps on the shins (erythema nodosum) or as deep, poorly healing ulcers (pyoderma gangrenosum). The European specialist society counts them among the typical manifestations outside the gut.

It also works the other way round: people with an inflammatory skin disease more often have a bowel disease. A meta-analysis Meta-analysis An analysis that combines the results of several studies. It is only as good as the studies it includes. of almost eight million people found that psoriasis came with about a 1.7-fold higher risk of ulcerative colitis and about a 2.5-fold higher risk of Crohn's disease. Shared genes and shared inflammatory pathways are discussed as explanations. In eczema the extra risk of bowel inflammation is smaller.

Disrupted state: causes and effects

Dermatitis herpetiformis is not hype but a skin form of coeliac disease Coeliac disease An autoimmune disease in which gluten damages the lining of the small intestine. It is diagnosed with a blood test and usually a biopsy while you are still eating gluten. More in the chapter “Lactose, fructose, gluten: making sense of intolerances”. It causes intensely itchy, clustered blisters, typically on the elbows, knees and buttocks. Gut symptoms are often absent, yet three in four affected people have villous damage in the small bowel. A meta-analysis found it in about 7 percent of people with coeliac disease, more often in adults than in children. Treatment is a lifelong gluten-free diet, at the start usually together with a medicine against itching and blisters.

Rosacea, the persistent facial redness with small spots, is linked to gut diseases in registries. In Denmark, people with rosacea more often had coeliac disease, Crohn's disease, ulcerative colitis and IBS Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis”. In an Italian study, 52 of 113 people with rosacea had small intestinal bacterial overgrowth Small intestinal bacterial overgrowth (SIBO) Too many bacteria in the small intestine, where normally few live. Bloating and diarrhoea are typical; it is usually diagnosed with a breath test. More in the chapter “When the small intestine is too densely colonised” on a breath test Breath test After drinking a sugar solution, hydrogen and methane in the breath are measured several times. This can reveal sugar intolerances and signs of bacterial overgrowth. More in the chapter “Gut tests: what they show and what they do not”, but only 3 of 60 healthy people. After treatment with the antibiotic rifaximin, rosacea cleared in 20 of 28, but not with placebo Placebo A dummy treatment without an active ingredient. Comparing with it shows how much a remedy achieves beyond expectation and chance. .

That sounds impressive but rests mainly on this one small study. In the Danish registry, rosacea was not linked to diagnosed bacterial overgrowth, and a meta-analysis found only a weak, uncertain link between rosacea and the stomach bacterium Helicobacter; treating it did not reliably improve rosacea.

With acne, diet and hormones matter more than gut bacteria. In a randomised trial Randomised trial A study in which chance decides who gets the treatment and who does not, for comparison. This makes results particularly reliable. , a diet with a low glycaemic load, that is fewer rapidly available carbohydrates, improved acne more than the comparison diet; but participants also lost weight. Observational studies link a lot of milk, especially skimmed milk, to more acne, but not yoghurt and cheese. Whether milk is the cause remains open.

In eczema (atopic dermatitis), probiotics Probiotics Live microorganisms that are meant to provide a benefit in sufficient amounts. The effect depends on the individual strain. More in the chapter “Using fermented foods well” have been well studied, with a sobering result: a Cochrane review of 39 trials concluded that available strains probably make little or no difference to symptoms. For prevention in infants the picture is somewhat more favourable, but uncertain (see Allergies and the gut).

Where the lines blur, the hype begins. There are no studies for "skin detox", gut cleanses against spots or stool tests meant to explain a skin disease. Nor is it proven in humans that a "leaky gut" is the common cause of all skin problems.